1. Field of the Invention
This invention relates to a recording head capable of accomplishing half tone expression in effecting recording on a recording medium and a recording apparatus provided with such recording head.
The term "recording apparatus" covers a printer, a facsimile apparatus, a copying apparatus, a word processor, an electronic typewriter, etc.
2. Related Background Art
A recording apparatus such as a printer or a facsimile apparatus is such that a dot pattern is formed on a recording sheet (a recording medium such as recording paper or a plastic sheet) while a plurality of dot forming elements provided on a recording head are selectively driven on the basis of recording information (image signals). As the types of such a recording apparatus, there are the serial type in which recording is effected while a recording head is moved widthwisely of a sheet, the line print type in which recording is effected collectively over a predetermined length in the direction of line, and the page print type in which recording is effected collectively for one page.
Also, the recording systems include the thermal system, the ink jet system, the wire dot system, etc. Of these, the thermal system can be classified into the heat transfer system in which ink is transferred to plain paper by the use of an ink sheet and the thermosensitive system in which thermosensitive paper is heated by a thermal head to cause color forming.
In these recording systems, a half tone recording method for expressing the density difference has heretofore been adopted during color recording or image recording using a plurality of colors such as cyan, magenta, yellow and black. In such conventional half tone recording, a method based on the principle of binary recording is generally adopted. As a technique for such harmonization expression, there has been adopted a technique of expressing half tone falsely by an area harmonization method such as the dither method in which with a plurality of dots as a unit, half tone is expressed by the rate of ON-OFF (two values) of the dots in the unit.
However, when the above-described area harmonization method is adopted, the number of dots necessary for one picture element increases in order to express many harmonies. This poses the problem that the resolution of image is reduced. To obtain an image of a resolution of the order of 6 picture elements/mm, for example, at 64 harmonies, by this area harmonization method, the resolution of the recording head need be of the order of 48 dots/mm. To realize this in a thermal printer, a thermal head of 48 dots/mm becomes necessary. However, the manufacture of a thermal head of such high density is difficult in the presentday technique. Even if such a thermal head could be manufactured, the number of elements will become huge and therefore, a large-scale driving circuit will become necessary for the driving of the thermal head, and this is not realistic. That is, in two-value recording, there is a limit in obtaining a harmony record of high image quality, and it has been desired that a multivalue harmony record expressing the size of a dot in multiple harmonies by some method or other be put into practical use.
Here, in the recording by heat melting transfer using a thermal head heretofore generally utilized, the density variation corresponding to a variation in applied energy is as shown in FIG. 9A of the accompanying drawings. That is, the rate of variation in the recording density to the applied energy E.sub.A is great, and the scattering width S.sub.W of the recording density D.sub.R is also great as indicated by the length of the vertical line in the graph. Therefore, it has been difficult to turn out intermediate density. In the prior-art thermal head shown in FIG. 9B of the accompanying drawings, a heat generating element 101 and electrodes 102 and 104 are of such a shape that they are of the same width and therefore, the distribution of an electric current flowing to the heat generating element 101 becomes uniform. The reference numeral 103 indicates the direction in which the electric current flows. Therefore, the temperature distribution of the heat generating element 101 is of such a degree that the temperature becomes somewhat high in the central portion of the heat generating element 101 wherein the amount of radiation is relatively small. Accordingly, even if the time of application of a pulse voltage applied to the signal electrode 104 is varied to thereby vary the temperature distribution of the heat generating element 101 as indicated by 111A and 112A (the time of application is 111A&lt;112A) in FIG. 9B, whether the temperature of the heat generating element 101 becomes higher than the melting point Tm of heat transfer ink is very subtle depending on the position thereof. Accordingly, even if the same applied energy is applied to the heat generating element 101 as shown in FIG. 9A, the recording density will differ depending on a slight difference in the position of the heat generating element. Therefore, the heat transfer recording method using the prior-art thermal head could virtually accomplish only two-value or binary recording, and an improvement in the reproducibility of harmony has been desired.
So, the applicant has proposed a thermal head of capable of multivalue recording in Japanese Laid-Open Patent Application No. 63-54261 (filed in Japan on Aug. 26, 1986 and laid open on Mar. 8, 1988). According to this, the width of an electrode at the junction between the electrode and a heat generating element is made less than the effective recording width of the heat generating element. Thereby, in a heat generating member, particularly that portion of the heat generating member which is near the junction with the electrode is caused to generate heat concentratedly, whereby said portion can be endowed with a heat generation distribution. However, even with such a construction, there has been the undesirable possibility of sufficient harmonization being not obtained when recording is effected on recording paper whose surface is not smooth, and a thermal head and a recording apparatus which are more excellent in harmonization have been desired.